Advances in medical imaging technology have lead to significant improvements in the capability of medical professionals to diagnose and treat diseases. The battle against cancer has particularly benefited from these advances. Medical imaging improvements have made tumors more visible, and therefore have increased rates of early detection. In addition, many treatments for cancer—such as adaptive radiation therapy—require that a medical professional have highly accurate information regarding the location of a tumor in a patient's body so that radiation can be applied to minimize damage to healthy tissue while applying a concentrated dose of radiation to the tumor. Through both of these channels, advances in medical imaging technology have increased the medical community's ability to combat diseases such as cancer, and save lives.
Modern medical imaging traces its roots to the turn of the twentieth century with the production of the first x-ray radiograph. A traditional radiograph consists of a two-dimensional image of a scanned three-dimensional object. Such two-dimensional images can only provide information regarding a single layer of the scanned three-dimensional object. However, advances in the fields of computing and image processing have enabled medical imaging technology to advance to a stage where full three-dimensional representations of a scanned object can be obtained and utilized by a medical professional. The resultant three-dimensional representations are called tomograms, and they are produced from a large set of two-dimensional projections that are obtained and combined by a process called computed tomography scanning.
Computed tomography scanning involves the use of a scanning procedure that is conducted using a scanning source and a scanning detector that are dispersed on either side of a patient. Usually both the scanning source and scanning detector are mounted on the gantry of a medical imaging apparatus. Once scanning begins, the scanning source sends some form of detectable penetrating signal through the patient's body, and the resultant signal that is detected by the scanning detector provides information regarding the portion of the patient's body that is being scanned. This information is used to create a set of projections. After each projection is obtained, the gantry will change position slightly such that the scanning source and scanning detector are positioned relative the patient's body at a slightly different angle. In this manner, each new projection obtained provides additional information regarding the full tomogram of the object. The combined tomogram provides a large amount of information to a medical practitioner regarding the condition of the portion of the patient's body that was scanned such as the presence of tumors, potential blood clots, bone fractures, or other problematic conditions.
Applying a computed tomography scanning procedure to a moving anatomical structure greatly increases the complexity of the problem. As described above, the location of the scanning source and scanning detector are adjusted between the acquisition of each projection. If the anatomical structure is moving, it will change shape and position during this adjustment period, and the next acquisition will obtain a projection of a slightly different object. When the resultant projections are used to reconstruct a tomogram, the tomogram will be an amalgam of the anatomical structure in different configurations and it will not accurately represent the object as it exists in the desired configuration. The tomographic cross sections obtained when the structure was not in the desired configuration will create what are called artifacts in the image, which degrades the information content of the tomogram. This problem is particularly harmful in the field of lung cancer tumor detection given that the human lung moves and deforms dramatically during respiration.
One approach to obtaining a tomogram of a moving anatomical structure is called gated computed tomography scanning. The addition of the term “gated” is used to denote a family of approaches where either the number of projections chosen for reconstruction, or the number of projections taken as a whole are limited to produce a more accurate representation of the moving structure. One example of gated computed tomography scanning can be described with the assistance of FIG. 1. In this particular form of gated computed tomography, a marker is placed on the thorax of a patient which is used to track their breathing. From this marker, respiratory signal 100 is obtained. Axis 101 displays time, and axis 102 displays a patient's respiratory signal R(t), where the local maxima represent when a patient has fully inhaled and the local minima represent when a patient has fully exhaled. In accordance with this family of procedures, respiratory signal 100 will be used to gate the time periods when the structure is in a specific position and the projections are accepted, and when the structure is not in a specific position and they are rejected.
Referring again to FIG. 1, axis 111 tracks time on same scale as axis 101 and the Zero intercept of axis 111 With the zero intercept of axis 101. Axis 112 tracks the angle α at which the scanning apparatus is positioned at any time, while line 110 illustrates the angle α of a constant speed gantry. The gating aspect of these procedures manifests through the fact that the scanning source is turned on, or the projections are accepted for final use, only during the times covered by regions 120, 121, and 122. In this example signal 100 is gated by amplitude level 123. Therefore the gating windows span the time when the amplitude signal 100 exceeds level 123. This gating can also be done based on the phase of the respiratory signal. Level 123 is chosen such that the anatomical structure will alter the least during the gated period. For example, a human lung that is performing respiration will deform throughout the respiratory cycle except for a brief pause at the top and bottom of respiration. By only utilizing projections obtained in this window of minimal movement at the top or bottom of the cycle, artifacts caused by the movement of the object are mitigated.
The problem of gated scanning is not completely handled simply by rejecting projections during some phases of movement. If some projections are missing, their absence will also cause artifacts. Therefore artifacts caused by motion corrupted projections cannot be solved simply by discarding such projections. With reference again to FIG. 1, regions 130 are those in which projections were captured. No projections were obtained for the remainder of axis 112 outside those regions. Regions for which projections were obtained are called spokes, and regions where no projections were obtained are called sparses. Complex computer algorithms are capable of extrapolating sparses given enough spokes. Certain algorithms can also extrapolate the movement of the structure and potentially ignore gating altogether. However, these algorithms are not perfect and it is always better to obtain actual images.
There are several current approaches for dealing with spokes in gated computed tomography scanning. First, the gantry can be set to continue to spin through several rotations until all of the spokes are eliminated. However, because a person's respiratory signal is not perfectly uniform, there is no way to assure that the scanning apparatus will be aligned to the desired a when the respiratory signal is in a gated region. Furthermore, although clinical accuracy of the images is important, many forms of scanning sources emit radiation that is harmful to patients. Therefore, minimizing scanning times is extremely important. Another current approach involves instructing a patient to hold their breath, or breathe according to a preset pattern during a scan. Although this approach can eliminate the misalignment problem and assure that all of the spokes are eliminated, it may be difficult for a patient to maintain the required breathing pattern. Given that a patient undergoing this form of analysis may potentially be suffering from lung cancer it may be especially difficult for them to maintain a desired breathing pattern.
Solutions to the problem of producing a tomogram of a moving object using computed tomography scanning will benefit the field of medicine and various other fields. In particular, motion agnostic medical image screening for possible lung tumors is of critical importance given the fact that lung cancer is the most rampant killer of all forms of cancer in the world. Given that traditional computed tomography scanning required multiple images of an object from many different angles that cannot be taken simultaneously, the application of computed tomography scanning to lung cancer detection is highly problematic. Key advances in this field must address the production of images with high information content while limiting the amount of radiation to the scanned object without having to artificially adjust the object's natural movement.